"Q: How many people go to church each Sunday? A: For years, the Gallup Research Organization has come up with a consistent figure — 40 percent of all Americans, or roughly 118 million people, who said they attended worship on the previous weekend. Recently, sociologists of religion have questioned that figure, saying Americans tend to exaggerate how often they attend. By actually counting the number of people who showed up at representative sample of churches, two researchers, Kirk Hadaway and Penny Marler found that only 20.4 percent of the population, or half the Gallup figure, attended church each weekend. As added proof for the accuracy of this smaller percentage of churchgoers, if 20.4% of Americans (approximately 63 million in 2010) attended the nation's 350,000 congregations weekly then the average attendance would be 180 people per congregation which is almost exactly the figure that numerous research studies have found."
Care with studies. Most Christians "just know" Christianity is falling away and pay big money to people who predict the death of the faith (George Barna AKA father of Christians divorce rate is the same or higher then non-Christian divorce rate though the one issue is his numbers were all a lie). The truth is that it has grown in mind blowing numbers and really hasn't slowed down except in Mainline denominations and Catholic Churches. The vast majority of stats are if people were honest to the question did they go to church. The surprise is most people are dishonest that they attend church weekly. Case in point East Sunday attendance is huge compared to normal weekly attendance.
I think people are just being blind to the meteoric rise of Christianity in the past 100 years. In 1910 more then half the Christians in the world were European.
Here is a good academic study. http://www.gordonconwell.edu/resources/Center-for-the-Study-...
"In 1990, 20.4% of the population attended an Orthodox Christian church on any given weekend. In 2000, that percentage dropped to 18.7% and to 17.7% by 2004. Olson explains that while church attendance numbers have stayed about the same from 1990 to 2004, the U.S. population has grown by 18.1% — more than 48 million people. "So even though the number of attendees is the same, our churches are not keeping up with population growth," he says." http://www.churchleaders.com/pastors/pastor-articles/139575-...
http://fastestgrowingreligion.com/numbers.html. Global Christian growth rate 1.36% is below the population groth rate of 1.41 %. Which is a sign of a slow death, Islam at 2.13% on the other hand is well above that.